Stability problems are known to be associated with the therapeutic oral administration of a protein (e.g., an antibody) or peptide intended to exert its therapeutic influence after passage through the stomach. More specifically, proteins or peptides tend to be degraded, or rendered therapeutically inactive, by the harsh environment of the gastrointestinal tract. The stomach is a characteristically low pH environment and the small intestine and colon contain a variety of gastric and intestinal proteases.
Hyperimmune bovine colostrum immunoglobulin (HBCIg) has been reported to be particularly resistant to gastrointestinal destruction, relative to other Igs. Early studies involving the administration of HBCIg for other gastrointestinal infections cited remarkable resistance to proteolytic digestion in the intestine of a heterologous host (Brussow et al., J. Clin. Microbiol. 25: 982-986 (1987)). Indeed there is evidence that bovine IgG1 is more resistant to proteolysis by trypsin, chymotrypsin and pepsin than other Igs (Butler, Vet. Immun. and Immunol. Pathology 4: 43-152 (1983); McClead and Gregory, Inf. and Immun. 44: 474-478 (1984)). Approximately 20% of immunoreactive bovine colostral Ig (as measured by radial immunodiffusion) was detectable in the ileal samples of human volunteers who consumed about 5 g of a .sup.15 N-labeled bovine colostral Ig preparation after fasting (Roos et al., J. of Nutrition 125: 1238-1244 (1995)). This recovery was cited as evidence of significant gastrointestinal resistance by comparison to the normal ileal digestibility of other milk proteins reported in the literature to be 90% or more. Unfortunately, the 20% immunoreactive Igs detected in this study were not shown to be functional (i.e., antigen binding).
Bovine Igs have also been cited as being particularly resistant to stomach acid (Rump et al., Clin. Invest. 70: 588-594 (1992)). Unfortunately, this assertion was based on exposure to "artificial gastric juice" (pH 2.4-4.0) for 24 hours. However, natural gastric juice is considered to have a pH of 1-2 (Kutchai, Gastrointestinal Systems, in Physiology 2nd ed: Berne and Levy (eds); 649-718; Mosby (St. Louis, Mo.) (1988)), not 2.4-4.0. Indeed, exposure of HBCIg to artificial gastric juice of pH 2.0 results in irreversible denaturation of antigen binding activity. Further, in studies of gnotobiotic piglets, even after pretreatment of the animals with cimetidine (which raised the mean gastric pH to 3.13), only 0.2-9% of the total activity of HBCIg was recovered in the stool (Tzipori et al., Clin. Diag. Lab. Immunol. 1: 450-463 (1994)). These data indicate that the majority of antibody activity is destroyed during passage through the gastrointestinal tract even with modified gastric pH, and that issues of formulation must be addressed to improve the proportion of active antibodies delivered via the oral route. The development of dosage formulations for oral immunotherapy must meet criteria of gastric resistance followed by complete release at the site of action (e.g., small intestine or colon).
The transit time of orally administered antibody varies considerably, and is dependent on the individual subject's physiology, fed or fasted status, type of food ingested, etc. In the fasted state, antibodies have a transit time through the small intestine of the order of 1-2 hours (Roos et al., J. of Nutrition 125: 1238-1244 (1995)). In contrast, systemic administration of antibodies would be expected to have a much longer duration of activity, as IgA has a half-life in circulation of about 6 days. Therefore, for sustained duration of activity, oral antibody therapy requires several doses per day, whereas systemic or injectable antibody therapy would require doses of the order of once per week.